1. Field of the Invention
The present invention relates to methods and apparatus for forming heads on the ends of metal rods, and more particularly, to methods and apparatus for hot forming or upsetting ends of snap tie rods into polygonal shapes.
2. Prior Art
Tie rods or "snap ties" are used to hold concrete form panels in place during the time the concrete is poured between the forms and hardens. Preferably, the tie rods are made of a metal rod having enlarged heads formed on its ends and a pair of notches formed along its length. To remove the protruding ends from the hardened concrete form, a tool such as a pair of pliers is used to grasp a head and twist it, which snaps the rod end off at the closest notched portion.
In order to minimize manufacturing cost of such tie rods, it is necessary to mass produce them. A well-know method of forming heads on the ends of a tie rod consists of gripping the rod near an end between halves of a split electrode with sufficient force to prevent relative movement, contacting the rod tip adjacent the end with a second electrode, heating the rod end by passing a current from one electrode through the rod end to the other electrode, and then urging the electrodes towards each other to deform or hot upset the end.
A disadvantage with this type of apparatus is that the electrodes do not shape the head as it is being formed. Consequently, the type of head formed has a generally round contour and is irregularly shaped. It often is not concentric with the axis of the tie rod. Such irregularly formed heads are difficult to grasp with tools at the time the end of the tie rod is to be snapped off the remainder, and the non-concentric position of the head increases the likelihood that the head itself is snapped off the tie rod when gripped by a tool, leaving a portion of the rod end protruding from the hardened concrete casting.
In order to provide a tie rod having regularly formed heads capable of being grasped by tools, the electrodes have been provided with recesses into which the rod ends are deformed during the upsetting process. For example, in U.S. Pat. No. 3,783,462, an apparatus for forming pentagonal heads on the ends of tie rods is disclosed. This apparatus comprises a gripper electrode having opposing jaws defining a central bore, and a reciprocating electrode having a recess defined by walls forming a pentagonal contour and a floor forming a heading tip. A tie rod is placed into the apparatus and gripped by the gripper electrode such that an end of the rod protrudes from the gripper electrode toward the reciprocating electrode. The recess in the reciprocating electrode is substantially concentric with the axis of the rod. The reciprocating electrode is brought into contact with the tip of the rod end, the rod end is heated by passing an electrical current through it from one electrode to the other, and the reciprocating electrode is brought into abutment with the gripper electrodes, thereby deforming the rod end into the pentagonal recess of the reciprocating electrode. The reciprocating electrode is retracted, the gripper electrodes are separated, and the tie rod having a pentagonal head is removed from the apparatus.
A problem inherent in apparatus and methods of the type previously described is that it is difficult to cause the heated rod end to flow into an electrode die recess having a polygonal shape such that it assumes the polygonal shape of the recess with sufficient accuracy to enable the finished head to be gripped by a tool such as a pliers or socket wrench. In particular, the metal which flows into the recess to form the head is under insufficient pressure to force it into the corners of the polygonal contour. The result is a head having a generally polygonal contour but with rounded corners. Efforts to increase the pressure acting upon the heated metal within the recess by forcing a greater volume of metal into the recess have resulted in excessive flash formed about the periphery of the finished head. Such flash is caused by the overflow of metal from the recess and makes it difficult if not impossible to grasp the formed head with a tool.
Accordingly, head upsetting apparatus having polygonal recesses have been limited to forming triangular, square, or pentagonal heads since only in these shapes are the angles formed by the intersection of the sides sufficiently accurately defined to enable the finished head to be grasped by a tool even though the corners of the head are rounded. Indeed, in the aforementioned patent, the statement is made that with heads having six or more sides, the finished head is so nearly round that it cannot be reliably gripped with a socket wrench.
An additional problem encountered in such apparatus utilizing die recesses having polygonal shapes is that due to the rapid cycle time and relatively loose tolerances of the electrodes, the tie rod is often positioned off-center prior to the heating and upsetting steps. As a result, the tie rod may be urged against a wall of the polygonal recess, resulting in a malformed tie rod head or the chipping of the die recess.
Accordingly, there is a need for a method and apparatus for forming polygonal heads on a tie rod in which the polygonal head formed is sufficiently accurate to be grasped by a tool with sufficient force to snap the end of the tie rod from the remainder without slippage of the tool with respect to the head. In addition, there is a need for an apparatus for forming polygonal tie rod heads which is capable of accurately locating the tie rod with respect to the die recess to prevent damage to the die recess during the forming operation.